Xiǎohán

Xiaohan, Minor Cold, is the twenty-third solar term, at the sun's longitude of 285 degrees, around the sixth of January. It is a sectional (jie) term. Despite its name it falls in one of the coldest stretches of the year, the severe cold that grips the land after the solstice. In the farming almanac it is a period of deep rest and endurance, with attention to keeping livestock and stored crops safe from the cold, and its customs centre on warming, fortifying foods. It falls near the approach of the lunar new year in many years. Being a solar marker, it comes near the same Gregorian date each year. These are the observances of the East Asian agricultural tradition, resting on the twenty-four-fold division of the tropical year into equal solar arcs.

Xiaohan, Minor Cold, falls at 285 degrees, around 5 or 6 January, and is a jie, or sectional, term, whose name signals the onset of the year's coldest stretch, continuing into Dahan, the major cold, roughly two weeks later. Despite its name, meteorological records for many parts of northern China in fact show Dahan, not Xiaohan, is often the milder of the two on average in some years, an inversion the traditional naming does not account for, since the terms were fixed by solar position established well before instrumental temperature records existed to confirm or challenge the expected sequence. Xiaohan traditionally opens the final stretch of the solar year before it returns to Lichun, the start of spring.

The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 285 degrees of the sun's apparent longitude. It is a sectional term, a jie, one of the twelve that fall in the odd positions of the twenty-four and open the twelve solar months. The sectional terms and the major terms alternate strictly around the year, so a jie is always followed by a zhong fifteen degrees later.

The terms have not always been spaced the same way. Until the seventeenth century they were reckoned by equal intervals of time, dividing the year into twenty-four equal parts, a method called pingqi. The Shixian calendar, prepared with Jesuit assistance and promulgated in 1645, redefined them by equal intervals of the sun's apparent longitude instead, fifteen degrees apart, which is the definition still used. Because the Earth's orbit is elliptical the sun does not move at a uniform apparent rate, so the intervals between terms now vary by about a day between the shortest and the longest.

Xiaohan, minor cold, opens the last pair of the twenty-four. The coldest weather of the year in northern China generally falls between this term and Dahan, so the two cold terms sit at the climatic low point in the same way the two heat terms sit at the high one, and the symmetry of the list about the solstices is close to exact.